This invention relates to a method and means for measuring the optical return loss in an optical transmission system.
Phenomenal growth of industry sectors such as the Internet and high-speed data, service providers are searching for ways to increase their fiber optic network capacity sooner than expected. Service providers are searching for new and creative ways to generate revenues while fully meeting the varying needs of their customers. Dense Wavelength Division Multiplexing (DWDM) partitions and maintains different dedicated wavelengths for different customers, for example, service providers can lease individual wavelengths as opposed to an entire fiber. Using optical multiplexers and optical amplifiers, DWDM combines multiple optical signals so that they can be amplified as a group and transported as wavelength channels over a single optical fiber to increase network capacity.
In the field of high capacity optical transmissions systems there is a need to provide a means for detecting the optical return loss. Optical return loss is defined as the proportion of reflected-optical signal measured at a given wavelength at the output of an amplifier circuit pack. Optical transmission systems propagate signals along multiple wavelength channels on a single medium. The signals, each having a different wavelength, may travel through different optical paths and equipment. Based on the optical path, the number of reflections per signal may differ. Isolating the return loss (RL) for a particular wavelength increases diagnostic capabilities of the optical transmission system. The diagnosis can report excessive return loss in the transmission of a signal at an individual wavelength.
Optical return loss may be detected at an increased level at the output of an optical amplifier, such as an erbium doped fiber amplifier (EDFA). The optical amplifier is charged with amplifying an optical signal. Amplifying a signal increases the data rate of the signal to a much higher rate along the optical communication path. As signals are amplified, reflections of a certain portion of light occur along a fiber optic path. These reflections are measured and used to determine the optical return loss.
Techniques to measure optical return loss (RL)have been disclosed. In U.S. Pat. No. 5,822,094, (O""Sullivan et al.), issued Oct. 13, 1998, and assigned to Nortel Networks, RL is measured over an average of wavelengths. This technique does not measure optical return loss on a per wavelength basis.
U.S. Pat. No. 6,111,676, (Avid Lemus et al.), issued Aug. 29, 2000 and assigned to Nortel Networks, discloses a means to measure optical return loss on a per wavelength basis. Although the invention does detect return loss, the detection means is limited. Lemus et al. describes a wavelength specific optical reflection meter/locator in signatured wavelength division multiplexed systems. This optical reflection meter/locator requires a unique intensity dither on each wavelength that is known to the optical amplifier. In the case of unknown wavelength sources, information regarding the intensity dither is not known or difficult to acquire. In addition, for high wavelength count and multiple span optical systems, cross-coupling of dithers from one wavelength to another will occur due to Stimulated Rayleigh Scattering which compromises the accuracy of the calculations of network loss.
The objective of any communication system is to offer continuous transmission service and, preferably, to detect and isolate faults. The process of detecting faults must be accomplished with minimum disruption or attenuation of the signal. With limited knowledge of the signal, the invention can detect the optical return loss. The present invention creates an accurate and efficient diagnosis of transmission inadequacies. In high capacity optical networks, companies must maintain certain transmission levels which include bounded optical return losses. If return losses are too low, then the signals can be degraded by multi-path interference. Low optical return losses at the output of optical amplifiers are particularly dangerous for multi-path effects, where light is reflected back into the amplifier, experiences gain, and is then reflected back out in the original direction. The delayed version of the optical signal can interfere with itself which causes optical performance degradation. If return losses are high, the signals being transmitted are degraded. This reduces the revenue generated by the transmission system services. As optical transmission systems service larger geographical distances in the network, manual diagnostics has become virtually impossible. The present invention eliminates the latter inefficiency.
The present invention provides an apparatus and method of detecting, on a per wavelength basis, optical return loss at the output of an optical circuit pack. The optical circuit pack can be, for example, an optical amplifier. At an output port and reflected port of the optical circuit pack a means for detecting signals is connected. The output power and reflected power are measured, and by the same means, the optical return loss is calculated. A return loss may be calculated for signals on one or more wavelength channels.
In one aspect the invention provides an apparatus for detecting optical return loss at the output port of an optical circuit pack in an optical transmission system, which comprises:
detecting means for detecting an outgoing optical signal of the optical circuit pack;
measuring means for measuring output wavelength power of an outgoing optical signal;
measuring means for measuring reflected wavelength power of a reflected optical signal for detecting optical return loss;
processing means for processing said detected outgoing optical signal for determining ratio of the reflected wavelength power to the output wavelength power, the ratio indicating the optical return loss;
wherein the detecting means outputs a detected signal to the measuring means, the measuring means outputs measured output wavelength power and reflected wavelength power to said processing means, the processing means determines the optical return loss calculated from measuring output wavelength power and reflected wavelength power.
In another aspect the invention provides a method for detecting return loss for a signal on a per wavelength basis at an output of an optical circuit pack, comprising the steps of
detecting an optical signal at the output of the optical circuit pack;
measuring power for an output signal and a reflected signal at an output port and reflected port respectively and an optical tap connected to a fused splitter at the output of the optical circuit pack; and
calculating return loss per wavelength, as a ratio of the reflected power per wavelength to the output power per wavelength.
Because of the recent drop in price of Optical Spectrum Analysers (OSA),their use is increasingly cost effective and practical in DWDM Systems. This has been due to the development of photo-detector array based OSA, as well as tuned Fabry-Perot filters. Cheaper OSAs make the above invention more amenable to implementation.